To equal the equivalent and same amount of a whole nutmeg you would use 3 teaspoons of ground nutmeg.
So if your recipe calls for a whole nutmeg that would be ground down to powder you would take and scoop up 3 teaspoons of ground nutmeg to equal one whole nutmeg.
If your recipe calls for freshly ground nutmeg and you don't have any whole nutmegs to ground down you can use already ground nutmeg in the recipe.
The freshly ground nutmeg tastes better in the recipes but you can use already ground nutmeg if that is all you have or you don't want to ground down your own nutmeg.
I prefer to ground my own fresh nutmegs into nutmeg powder but you can buy already ground nutmeg to use in the recipes if you prefer to do so.